Research news

Loss of telomere function can induce cell cycle arrest and apoptosis but the processes
that trigger cellular responses to telomere dysfunction remain largely unknown. In
the October 5 issue of Cell, Michael Hemann and colleagues from Johns Hopkins University School of Medicine show that the shortest telomere, not average telomere length, is critical for cell
viability and chromosome stability (Cell 2001, 107:67-77).

Hemann et al. crossed telomerase-deficient mice having short telomeres with mice heterozygous for
telomerase activity, which have longer telomeres. They found that the phenotype of
the telomerase-null offspring was similar to that of the late generation parent, although
only half of the chromosomes were short. In addition, spectral karyotyping showed
that loss of telomere function occurred preferentially on chromosomes with critically
short telomeres.

"Our evidence suggests that once a telomere becomes very short, the cell recognizes
it as a DNA break," said Carol Greider the senior author of the paper. The break signals
the cell to enter apoptosis, as a protective mechanism to prevent chromosome rearrangement
and the development of cancer.

These results hint that the main task of telomerase is not to maintain average telomere
length but rather to maintain function for critically short telomeres.